I have this problem a lot -- I attribute it to the fact that my build is very elliptical. I have wide hips and big upper thighs, but do not hold weight in my tummy and I have a flattish butt. Therefore, cuts that accommodate my hips and thighs often have a little too much room in both the waist and butt. It is possible to tailor for both issues, but I usually just have waists taken in and hope for the best when it comes to the rear view. Certain fabric contents seem to lead to bagging out more than others, but I have yet to find a magic fabrication -- some that seem promising at first behave differently after the first laundering, while others spring back to shape during wear with no problems.